Yes, i have done all of that steve, tried more new batteries still no more than 3 v registering - could the chip be at fault, looks easy enough to change out as it has the plug type holder.
I probably wouldn't change the chip at this stage.
Try checking the voltages at the battery holder leads where they enter the board, then at the blocking diode (D17) which is directly above the 5101 Ram chip.
See here for location of this diode if you are unsure.
http://pinball.flippers.info/system6mpulowres.pdf
